There are numerous applications for which an economical and efficient device and method for continuously monitoring vapor densities, composition, flow velocity, internal and kinetic temperatures and constituent distributions is needed.
One such application is the Atomic Vapor Laser Isotope Separation (AVLIS) process. In the AVLIS process, uranium is vaporized and the U.sup.235 isotope is selectively photo-ionized for subsequent electrostatic collection. This process is shown schematically in FIG. 1. A high power electron beam system is used to co-vaporize a mixture of uranium and iron. The economics of the AVLIS process is governed by the vaporization rate, the distribution of internal states of uranium, the collection efficiency and the component lifetime. Each of these factors are critically dependent on the specific properties of the vapor. The vaporization rate and internal temperature are directly measurable, while the collection efficiency is dependent on the flow velocity and flow circulation as well as the flux distribution. Likewise, the component lifetime in a corrosive liquid uranium environment depends on the component temperature. By vaporizing a mixture of uranium and iron, an eutectic alloy is formed having a lower melting temperature. This enables a lower operating temperature which, in turn, is more economical. The temperature necessary to maintain liquid flow on each component depends on the ratio of iron to uranium on that component. Thus, in order to optimize the AVLIS process, it is necessary to closely control the composition of the uranium-iron vapor.
Electron beam vaporization processes also represent an area of technology that would greatly benefit from a process enabling the continuous monitoring of vapor properties. One application of electron beam vaporization that would benefit from improved vapor monitoring technology is rapid prototyping with an electron beam vaporizer. Numerous alloys are comprised of elements with vastly different vapor pressures. For example, the vapor pressures of iron and molybdenum for 316 stainless steel differ by more than three orders of magnitude at 3000.degree. K. As a result of this large vapor pressure difference, compositional variation is common. A significant need exists for a device and method capable of closely regulating the composition of the metal vapor.
The formation of titanium alloys for aerospace parts is another technology where closer regulation of vapor composition is needed. D. Hughs, Aviation Week and Space Technology (1991) p. 358. In the absence of a means for continuously monitoring the vapor phase, the aerospace industry currently must rely on empirical recipes to produce titanium alloys for aerospace parts. However, the rejection rate for aerospace alloys formed using empirical recipes presently exceeds 10%. By continuously monitoring the metal composition of the vapor forming the alloy, it should be possible to better control the resulting alloy composition and reduce the current rejection rate.
A simple method for continuously monitoring the presence and density of a particular component of a gas vapor could also be used to effectively monitor compliance with environmental regulations including in situ trace gas detection, waste stream analysis, atmospheric measurements and vehicle emissions monitoring. Additional applications for an improved method and device for monitoring vapor composition and properties include but are not limited to plasma etching of semiconductors, thin film deposition control and molecular beam and atomic layer epitaxy.
Several methods presently exist for determining the density of a particular component of a gas vapor. However, these methods are generally expensive, intrusive and lack the necessary accuracy, reliability and durability. For example, direct deposition sensors have been used to measure the total vapor flux onto a surface, independent of chemical composition. These sensors are commonly used for inferring the thickness of a deposited film. Examples of direct deposition sensors include load cells, quartz crystal micro-balances and chopped ion gauges.
Direct deposition sensors operate based on direct deposition of the component being measured on to the sensor and thus must be placed within the gas sample. These sensors are not able to distinguish between different deposited components and thus cannot be used to detect the density of individual components in a co-vaporization process. Further, because direct deposition sensors are intrusive, these sensors must be able to withstand extreme temperatures, often in excess of (1000.degree. C.) and can disrupt the vapor plume of the process being monitored. In addition, because these sensors are exposed to extreme temperatures and operate by direct deposition of the component being measured onto the sensor, direct deposition sensors generally have short operating lifetimes. For example, quartz crystal micro-balance sensors generally do not have operating lifetimes in excess of 5 hours for high vaporization rate processes.
Emission spectroscopy has also been used to monitor the density of particular components of a gas vapor. Examples of emission spectroscopy systems include electron impact emission and x-ray spectrometers. Emission spectroscopy has relatively poor spectral resolution (&gt;10 GHz) which makes it difficult to resolve similar components. In addition, emission spectroscopy requires the use of a vapor heating source which adversely affects vapor properties. Further, the diagnostic probe is necessarily subjected to the harsh environment internal to the vaporizer vessel, thereby reducing the lifetime and reliability of the diagnostic.
Atomic absorption spectroscopy has also been used to monitor the density of particular components in gas vapor samples. Atomic absorption spectroscopy has the advantage of being species specific. However, the usefulness of atomic absorption spectroscopy is largely limited by the quality of the light source. Examples of atomic absorption spectroscopy include spectral lamp and ring-dye laser based systems.
Spectral lamps are inherently low light level, broad-band, incoherent sources. As a result, in order to gather enough photons to make a density measurement, short pathlengths are necessary. Therefore, it is generally necessary to place the launch optics within the gas sample being tested. This greatly reduces the reliability and durability of spectral lamp components since they are often exposed to high temperature gas samples. In addition, the intrusive nature of spectral lamp atomic absorption spectroscopy can disrupt the vapor plume.
Spectral lamps also require recalibration any time an operating parameter or diagnostic location is altered. The need to continuously recalibrate spectral lamps arises from the fact that the emission band of spectral lamps is broad (1 GHz) and untunable. The monochromators that are likely to be used to measure vapor compositions have a broad frequency acceptance window. As a result, a narrow spectral absorption feature will have little effect on the output signal since the majority of the light transmitted from the hollow cathode will not be affected. Vapor sources that have a doppler broadened width close to that of the hollow cathode are therefore difficult to measure. The sensitivity of the photo-multiplier to significant absorption of light in the tails of the hollow cathode emission curve is much smaller than at the center of the emission curve.
Ring dye lasers have also been used in atomic absorption spectroscopy. Ring dye lasers are advantageous over spectral lamps in atomic absorption spectroscopy in several respects. Ring dye lasers provide high level coherent light. As a result, ring dye lasers enable longer pathlengths and can thus generally be employed nonintrusively. In addition, ring dye lasers enable significantly enhanced spectral resolution (1 MHz) thus enabling element specific and, in some cases, isotope specific detection. In addition, ring dye lasers are tunable over a broad wavelength range to 60 nm. In view of these advantages, ring dye laser atom absorption spectrometry is a standard technique used in the laboratory for element and isotope specific identification and quantitative measurement.
Despite its prevalent use, ring dye laser based atomic absorption spectrometry has several significant disadvantages. Ring dye lasers are very expensive, costing around $160,000 with an operating cost of around $15,000 per 1000 hours to replace the laser ion tube. Further, weekly maintenance of the ring dye laser atomic absorption spectrometer is required. In addition to being costly to operate, atomic ring dye laser atomic absorption spectrometers are large and thus are not generally portable. An additional disadvantage of ring dye laser systems is the handling and disposal of mutagenic and carcinogenic dyes as well as flammable solvents. Ring dye lasers also require a special power supply (480 V 3 phase) and water cooling.
In view of the disadvantages associated with these prior art devices, the need still exists for a simple, accurate and economical device for evaluating the density of a particular component in a gas vapor sample. All of the disadvantages noted in these prior art devices have been overcome by the Diode Laser Vapor Density Monitor of the present invention.